1. Field of the Invention
The invention is in the field of subsurface mining of oil shale. More particularly the invention relates to an improvement in the underground mining of oil shale whereby that portion of the underground shale deposit that is normally left unmined for structural support purposes is recovered.
2. Description of the Prior Art
When underground oil shale deposits are mined it is necessary to provide structural support for the roof of the mine cavity. One way of providing such support is to leave vertical segments or pillars of unmined shale within the cavity to support the roof. These pillars often account for 30% to 40% by volume of the entire shale deposit. Leaving such a high proportion of the deposit unmined obviously detracts from the efficiency and economy of the underground shale mining operation. It would, therefore, be highly desirable to be able to remove these pillars and recover the kerogen therefrom. One obvious solution to this problem is to install support beams of wood or metal in the cavity and then remove the pillars. This solution, while workable, is not practical because the cost of obtaining such beams, transporting them to the mine site--which is typically in a remote area--and installing the beams in the mine cavity would far outweigh the incremental income realized by mining and retorting the pillars.
A principal object of the present invention is to provide an effective and relatively inexpensive technique for supporting the roof of underground oil shale mines to permit removal of the entire shale deposit. The technique uses spent oil shale--a material that is normally in ready and proximate supply--to form alternative support members in the mine. In this regard cementitious spent shale composites have been used previously in the construction field to make liners and caps for spent shale containment basins and to make linings for irrigation ditches and dams. Applicant knows of no use or suggested use of spent oil shale in making heavy vertical support members.